By George S. Bower

The Volt has a very sophisticated thermal management system. The high voltage battery temperature band is strictly controlled with an active liquid cooling system AND the battery only uses 65 percent of its capacity which should result in a long life.

An excellent discussion of the TMS system written by our own WopOnTour is given here . A schematic of the battery cooling system from that article is presented below in figure 1


What is the Volt Battery TMS Temperature Band?


We have speculated here that the temperature band was 68 F to 72 F which is an ideal range for Lithium batteries. In order to find that temperature band, data was taken using DashDAQ data acquisition device on a hot summer day while charging. Prior to charging, the Volt was parked in full sun for 3 hours and then moved into the garage for charging.

The TMS data taken during charging is presented in figures 2 thru 4.


We see from these figures that the Volt’s TMS switches on at 86 F and cools the Volt’s battery via the cars electric A/C compressor to 72 F and then shuts down. This 72 F to 86 F temperature band is somewhat higher than we had speculated.

What Happens if the Volt is Parked all Day in Full Sun?


What happens if the Volt is parked in the hot sun with ignition (power) off and NOT plugged in? This would be the scenario for many Arizona workers that park their cars in full sun on a black asphalt parking lot all day. How hot will the Volt’s battery get?

In order to determine, Volt was parked for a day in full sun. Results of this test are presented in figure 5. We see that the Volt’s battery is extremely well insulated. With cabin temperatures soaring to 115 F and ambient air temperatures going to 99 F the Volts battery only reached a maximum temperature of 90 F. Roughly equal to the upper limit during active thermal management during charging shown in figure 2 . We also note that there is NO ACTION taken by the TMS in this case.


It occurred to me that perhaps there was no action by TMS because the battery was at a 5-percent SOC. Therefore I repeated the test with the battery at 81-percent SOC and the results were unchanged ie there was NO ACTION taken by TMS and battery temp reached 91 F.

How hot would the Volts battery get if ambient air temperature reached 110-120 F as is common at the peak of summer? The answer is that we do not know yet. I do not have data at these extreme temperatures so all we can do at this point is SPECULATE. If the TMS system stays inactive at these high temps, then we could see battery temperatures of 100+ degrees at the peak of summer.

What Happens if we Leave the Ignition (power) on?


In the referenced thread it was suggested that one might mitigate battery temperatures by leaving the Volt powered on.
Data was taken during a hot soak with power ON (and NOT plugged in) and data was recorded using DashDAQ. The data is presented in figures 6 thru 8.

We see the following from these figures

1. Coolant loop pump and cooling fan were on the entire time.
2. A/C chiller cycled on and off maintaining the battery between 73 F and 90 F.
3. This resulted in a substantial loss in battery charge of 30 percent.
4. The flow control valve cycled between radiator (position C in schematic figure 1) and Chiller (see figure 7 and figure 1).

This flow control valve programming is unexplained at this point in time. Since outside air temperature was hotter than battery coolant temperature this actually resulted in heating the battery faster than when the ignition OFF hot soak. Note that during the charging test presented in figures 2 thru 4 the flow control valve cycled between BYPASS and Chill as would be expected. GM must have a reason for this programming but I do know what that reason is.

As a bottom line, leaving the Volt powered on all day in the parking lot DOES NOT seem like a prudent thing to do. I would think that a short power on (remote engine start) would be OK but that to only leave power on long enough to cool the battery via A/C chiller and then turn power off.

What to Do?


If the predicted (ambient) high temperature is less than 100 F don’t do anything.
If the predicted high is higher than 100 F, still not to worry. If you were supposed to worry about it GM would have provided a read out of battery temperature to the general public. As said earlier I don’t have battery temperature data above 100 F anyway (stay tuned).

However, if you are a conservative type, it wouldn’t hurt to take some precautionary measures on super hot days.

The following steps would help mitigate battery temperatures on days where the predicted high is above 100F:

1. Park in the shade.
2. Use window shade screens.
3. Plug in if possible.
4. Perhaps do a remote start late in the afternoon. (Caution engine could come on). However, I would only do the remote start long enough to cool the battery and then discontinue remote start for reasons discussed above.